Mass spectrometer

ABSTRACT

A mass spectrometer is disclosed comprising a guide wire ion guide  1  having an outer cylindrical electrode  2  and an inner guide wire electrode  3 . AC and DC potential differences are maintained between the outer electrode  2  and the inner electrode  3  so that ions are radially confined within the ion guide  1  in an annular potential well. The outer electrode  2  may be segmented and axial potential wells created along the length of the ion guide  1  may be translated along the length of the ion guide  1  by applying additional transient DC potentials to the segments forming the outer electrode  2.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from United Kingdom patent applicationGB-0220450.1 filed 3 Sep. 2002 and U.S. Provisional Application60/427,557 filed 20 Nov. 2002. The contents of these applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a mass spectrometer and a method ofmass spectrometry.

BACKGROUND OF THE INVENTION

Ion guides are known which are used to transport ions between differentregions in a mass spectrometer. For example, an ion guide may be used totransport ions from or to an ion source, collision cell, mass analyseror between regions having different gas pressures. Ion guides may alsobe used as gas cells to collisionally cool or heat continuous beams orpackets of ions by colliding the ions with a gas. Collisional coolingreduces the average kinetic energy of the ions which is advantageous,for example, for subsequent mass analysis of the ions using a Time ofFlight (“TOF”) mass analyser. Alternatively, ions may be collisionallyheated within an ion guide during transportation between two regions soas to cause the ions to fragment. The product, daughter or fragment ionsmay be mass analysed in order to determine the chemical structure of theassociated parent ions.

Conventional ion guides may comprise a multipole parallel rod set ofelectrodes e.g. a quadrupole, hexapole or higher order rod set or astacked concentric circular ring set of electrodes (i.e. an “ion tunnel”ion guide) comprising a plurality of electrodes having apertures throughwhich ions are transmitted in use. AC or RF voltages are applied toopposing rods in a multipole rod set or to alternate rings in an iontunnel ion guide such that the voltages applied to the opposing rods oralternate rings have opposite phases. The geometries of the electrodesin a multipole rod set or a ring set ion guide are arranged so thatinhomogeneous AC/RF electric fields generate pseudo-potential wells orchannels within the ion guide. The ions are preferably confined in thesepotential wells and are guided through the ion guide.

A significant issue with multipole rod set ion guides such asquadrupole, hexapole or octopole rod sets is that they are relativelycomplex arrangements and hence are comparatively expensive tomanufacture. The complexity and expense becomes a particularlysignificant problem if the multipole rod set ion guide is intended totransport ions over a relatively long distance.

Another known form of ion guide is an Electrostatic Particle Guide(“EPG”) which comprises a cylindrical electrode having a guide wirerunning along the central axis of the cylinder. Different static DCvoltages may be applied to the guide wire and the conductive outercylindrical electrode so that, for example, the guide wire may beconnected to a DC potential which attracts ions and the outercylindrical electrode may be connected to a DC potential which repelsions. Injected ions will follow elliptical paths around the guide wireunder conditions of high vacuum otherwise the velocity of the ions wouldbe dampened by collisions with gas molecules and the ions woulddischarge upon hitting the guide wire. The potential difference betweenthe guide wire and the outer cylindrical electrode generates a steeplogarithmic potential well within the ion guide with the centre of thepotential well being located at the guide wire. The guide wire may, forpositively charged ions, be at a lower potential than the outercylindrical electrode so that positive ions are attracted radiallyinwards towards the guide wire electrode. Negatively charged ions withinthe electrostatic particle guide will be attracted towards the outercylindrical electrode and will be lost. Alternatively, the guide wiremay be maintained at a higher potential relative to the outercylindrical electrode so that negative ions are attracted radiallyinwards towards the guide wire and positively charged ions are repelled.

Some of the positive or negative ions which are attracted to the guidewire enter into stable orbits about the guide wire along the length ofthe ion guide. However, other ions will strike the guide wire and willbe lost. The transmission losses due to ion collisions with the guidewire will depend upon the radius of the guide wire and the energy andspatial distribution of ions entering the guide wire ion guide.Significant transmission losses will occur when ions have kineticenergies in the radial direction which are greater than the depth of thepotential well within the cylindrical electrode. These energetic ionswill tend to strike the inner surface of the cylindrical electrode andwill become neutralised and lost. Further significant transmissionlosses are also observed if the conventional guide wire ion guide isoperated at relatively high pressures. At higher pressures the mean freepath between collisions between ions and neutral gas molecules issignificantly shorter than the length of the guide wire ion guide andhence the ions will tend to collide with the gas molecules many timesbefore leaving the ion guide. These collisions cause the ions to losekinetic energy which results in the ions spiraling into the guide wireand thus being lost.

In view of the above mentioned problems, known guide wire ion guides areonly used to transport ions through regions of relatively low gaspressure wherein collisions between ions and gas molecules are unlikely.

It is therefore desired to provide an improved guide wire ion guide andin particular a guide wire ion guide which is suitable for use atrelatively high pressures.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a massspectrometer comprising an ion guide having an outer electrode and aninner electrode disposed within the outer electrode. In use the innerand outer electrodes are maintained at a DC potential difference suchthat ions experience a first radial force towards the inner electrode.An AC or RF voltage is also applied to the inner and/or the outerelectrodes so that ions experience a second radial force towards theouter electrode.

In a preferred embodiment the AC or RF voltage is a single phase AC orRF voltage applied to the inner or outer electrode. Alternatively, theAC or RF voltage may comprise a two phase AC or RF voltage wherein afirst phase is applied to the inner electrode and a second oppositephase is applied to the outer electrode. Preferably, the AC or RFvoltage has a frequency of <100 kHz, 100-200 kHz, 200-300 kHz, 300-400kHz, 400-500 kHz, 0.5-1.0 MHz, 1.0-1.5 MHz, 1.5-2.0 MHz, 2.0-2.5 MHz,2.5-3.0 MHz, 3.0-3.5 MHz, 3.5-4.0 MHz, 4.0-4.5 MHz, 4.5-5.0 MHz, 5.0-5.5MHz, 5.5-6.0 MHz, 6.0-6.5 MHz, 6.5-7.0 MHz, 7.0-7.5 MHz, 7.5-8.0 MHz,8.0-8.5 MHz, 8.5-9.0 MHz, 9.0-9.5 MHz, 9.5-10.0 MHz or >10.0 MHz. Theamplitude of the AC or RF voltage is preferably <50 V peak to peak,50-100 V peak to peak, 100-150 V peak to peak, 150-200 V peak to peak,200-300 V peak to peak, 300-400 V peak to peak, 400-500 V peak to peak,500-600 V peak to peak, 600-700 V peak to peak, 700-800 V peak to peak,800-900 V peak to peak, 900-1000 V peak to peak, 1000-1100 V peak topeak, 1100-1200 V peak to peak, 1200-1300 V peak to peak, 1300-1400 Vpeak to peak, 1400-1500 V peak to peak or >1500 V peak to peak.

In one embodiment the timing of pulses of ions being directed into theion guide may be phase locked as synchronised with the AC/RF voltagesapplied to the electrodes. Ions may, for example, be arranged to enterthe ion guide according to the preferred embodiment as the AC/RF voltagepasses through zero. Alternatively, the phase may be locked so that theAc or RF voltage is not passing through zero as the ions enter the ionguide. For example, the AC/RF voltage may be arranged such that whenions enter the preferred ion guide the AC/RF electric field has amagnitude which creates a relatively large force on the ions in adirection towards the outer electrode. In this manner ions whichinitially enter the ion guide at an angle towards the inner electrodewill not travel too close to the inner electrode and hence will notsubstantially pick up as much radial kinetic energy from the AC/RFelectric field. Accordingly, ions initially travelling towards the innerelectrode will be more stable in the ion guide and hence will be morelikely to be transmitted from the entrance to the exit of the ion guide.

Preferably, the outer or inner electrode is maintained, in use, at a DCpotential <−500 V, −500 to −400 V, −400 to −300 V, −300 to −200 V, −200to −100 V, −100 to −75 V, −75 to −50 V, −50 to −25 V, −25 to 0V, 0V,0-25 V, 25-50 V, 50-75 V, 75-100 V, 100-200 V, 200-300 V, 300-400 V,400-500 V or >500 V. The DC potential difference between the outerelectrode and the inner electrode may be maintained, in use, at apotential difference 0.1-5 V, 5-10 V, 10-15 V, 15-20 V, 20-25 V, 25-30V, 30-40 V, 40-50 V, and >50 V, −0.1 to −5 V, −5 to −10 V, −10 to −15 V,−15 to −20 V, −20 to −25 V, −25 to −30 V, −30 to −40 V, −40 to −50 V or<−50 V.

In a preferred embodiment the inner electrode comprises a guide wire. Atleast 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of theinner electrode may comprise a semiconductor or resistive wire and inuse, an axial DC potential gradient may be maintained along at least10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of the innerelectrode by applying a DC potential difference across 10%, 20%, 30%,40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of the inner electrode.

In a further embodiment the inner electrode may comprise a cylindricalelectrode or a plurality of concentric cylindrical electrodes. An axialDC potential gradient may be maintained along at least 10%, 20%, 30%,40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of the inner electrode bymaintaining at least some of the plurality of concentric cylindricalelectrodes at different DC potentials.

In a preferred embodiment the inner and/or outer electrode comprise aplurality of electrodes such that in a mode of operation an axial DCpotential gradient may be maintained along at least 10%, 20%, 30%, 40%,50%, 60%, 70%, 80%, 90%, 95% or 100% of the length of the inner and/orouter electrode so that ions are urged along at least a portion of theion guide. The axial DC potential gradient may be maintainedsubstantially constant with time as ions pass along the ion guide.Alternatively, the axial DC potential gradient may vary with time asions pass along the ion guide.

The ion guide may comprise 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or >30segments, wherein each segment comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,29, 30 or >30 electrodes. The electrodes in each segment or a pluralityof segments are preferably maintained at substantially the same DCpotential. Each segment may be maintained at substantially the same DCpotential as the subsequent nth segment wherein n is 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,27, 28, 29, 30 or >30.

In a preferred embodiment, ions are constrained axially within the ionguide by a real potential barrier or well. Preferably, the transit timeof ions through the ion guide is selected from the group consisting of:less than or equal to 20 ms, less than or equal to 10 ms, less than orequal to 5 ms, less than or equal to 1 ms, and less than or equal to 0.5ms.

In a further embodiment, one or more transient DC voltages or one ormore transient DC voltage waveforms may be initially provided at a firstaxial position and may then subsequently be provided at second, thenthird different axial positions along the ion guide. The one or moretransient DC voltages or one or more transient DC voltage waveforms maymove from one end of the ion guide to another end of the ion guide sothat ions are urged along the ion guide. Preferably, the one or moretransient DC voltages create a potential hill or barrier, a potentialwell, multiple potential hills or barriers, multiple potential wells, acombination of a potential hill or barrier and a potential well, or acombination of multiple potential hills or barriers and multiplepotential wells. The one or more transient DC voltage waveforms maycomprise a repeating waveform, such as a square wave. The amplitude ofthe one or more transient DC voltages or the one or more transient DCvoltage waveforms may remain substantially constant or may vary withtime. The amplitude of the one or more transient DC voltages or the oneor more transient DC voltage waveforms may increase with time, increasethen decrease with time, decrease with time or decrease then increasewith time.

In a preferred embodiment the ion guide may comprise an upstreamentrance region, a downstream exit region and an intermediate region. Inthe entrance region, intermediate region and exit region the amplitudeof the one or more transient DC voltages or the one or more transient DCvoltage waveforms may have a first amplitude, second amplitude and thirdamplitude respectively. The entrance and/or exit region may comprise<5%; 5-10%, 10-15%, 15-20%, 20-25%, 25-30%, 30-35%, 35-40% or 40-45% ofthe total axial length of the ion guide. Preferably, the first and/orthird amplitudes are substantially zero and the second amplitude issubstantially non-zero. The second amplitude may be larger than thefirst and/or third amplitudes.

In a further embodiment the one or more transient DC voltages or the oneor more transient DC voltage waveforms pass along the ion guide with afirst velocity. The first velocity may either remain substantiallyconstant, vary, increase, increase then decrease, decrease, decreasethen increase, reduce to substantially zero, reverse direction, orreduce to substantially zero and then reverse direction. The one or moretransient DC voltages or the one or more transient DC voltage waveformspreferably causes ions within the ion guide to pass along the ion guidewith a second velocity. The first velocity and the second velocity maybe substantially the same. The first and second velocities may differ byless than or equal to 100 m/s, 90 m/s, 80 m/s, 70 m/s, 60 m/s, 50 m/s,40 m/s, 30 m/s, 20 m/s, 10 m/s, 5 m/s or 1 m/s. The first and/or secondvelocities may be 10-250 m/s, 250-500 m/s, 500-750 m/s, 750-1000 m/s,1000-1250 m/s, 1250-1500 m/s, 1500-1750 m/s, 1750-2000 m/s, 2000-2250m/s, 2250-2500 m/s, 2500-2750 m/s or 2750-3000 m/s.

In a preferred embodiment the one or more transient DC voltages or theone or more transient DC voltage waveforms may have a frequency orwavelength which remains substantially constant, varies, increases,increases then decreases, decreases, or decreases then increases.

In yet a further embodiment two or more transient DC voltages or two ormore transient DC voltage waveforms may pass substantiallysimultaneously along the ion guide. The two or more transient DCvoltages or waveforms may be arranged to move in the same direction, inopposite directions, towards each other or away from each other. One ormore of the transient DC voltages or waveforms may be repeatedlygenerated and passed along the ion guide. The frequency of generatingthe one or more transient DC voltages or waveforms may remainsubstantially constant, vary, increase, increase then decrease,decrease, or decrease then increase.

In another embodiment the mass spectrometer may comprise an ion detectorwhich is arranged to be substantially phase locked with pulses of ionsemerging from the exit of the ion guide. The mass spectrometer mayfurther or instead comprise a Time of Flight mass analyser comprising anelectrode for injecting ions into a drift or flight region, theelectrode being arranged to be energised in a substantially synchronisedmanner with the pulses of ions emerging from the exit of the ion guide.The mass spectrometer may further or instead comprise an ion traparranged downstream of the ion guide, the ion trap being arranged tostore and/or release ions from the ion trap in a substantiallysynchronised manner with pulses of ions emerging from the exit of theion guide. The mass spectrometer may further comprises a mass filterarranged downstream of the ion guide. A mass to charge ratiotransmission window of the mass filter may be varied in a substantiallysynchronised manner with pulses of ions emerging from the exit of theion guide in order to select ions having a particular charge state.Pulses of ions entering the ion guide may also be synchronised with thetransient DC potentials or waveforms.

In another embodiment the ion guide may comprise one, two, or more thantwo entrances for receiving ions and one, two, or more than two exitsfrom which ions emerge from the ion guide. The inner and/or outerelectrode may also be substantially Y-shaped.

In yet a further embodiment the ion guide comprises at least oneentrance for receiving ions along a first axis and at least one exitfrom which ions emerge from the ion guide along a second axis, whereinthe outer electrode and/or the inner electrode are curved between theentrance and the exit. The ion guide may, for example, be substantially“S”-shaped and/or have a single point of inflexion. The second axis mayalso be laterally displaced from the first axis. The second axis may beinclined at an angle θ to the first axis, wherein θ>0°. Preferably, θfalls within the range <10°, 10-20°, 20-30°, 30-40°, 40-50°, 50-60°,60-70°, 70-80°, 80-90°, 90-100°, 100-110°, 110-120°, 120-130°, 130-140°,140-150°, 150-160°, 160-170° or 170-180°.

The preferred ion guide may also have at least a portion which varies insize and/or shape along the length of the ion guide, or may have a widthand/or height which progressively tapers in size.

In a less preferred embodiment the ion guide may comprise an innerelectrode which is arranged offset from the central axis of the outerelectrode. The distance between the inner electrode and the outerelectrode may vary along at least a portion of the ion guide.

The mass spectrometer preferably comprises an Electrospray (“ESI”) ionsource, an Atmospheric Pressure Chemical Ionisation (“APCI”) ion source,an Atmospheric Pressure Photo Ionisation (“APPI”) ion source, a MatrixAssisted Laser Desorption Ionisation (“MALDI”) ion source, a LaserDesorption Ionisation (“LDI”) ion source, an Inductively Coupled Plasma(“ICP”) ion source, an Electron Impact (“EI”) ion source, a ChemicalIonisation (“CI”) ion source, a Fast Atom Bombardment (“FAB”) ion sourceor a Liquid Secondary Ions Mass Spectrometry (“LSIMS”) ion source. Theion source may be pulsed or continuous.

In a further embodiment the entrance and/or exit of the ion guide ismaintained at a potential so that ions are reflected at the entranceand/or exit of the ion guide. At least one ring lens, plate electrode orgrid electrode may be arranged at the entrance and/or exit of the ionguide and may be maintained at a potential so that ions are reflected atthe entrance and/or exit of the ion guide. An AC or RF voltage and/or aDC voltage may be supplied to the at least one ring lens, plateelectrode or grid electrode so that ions are reflected at the entranceand/or exit of the ion guide.

In a preferred embodiment the mass spectrometer further comprises a massanalyser arranged downstream of the ion guide. The mass analyser may,for example, comprise a Time of Flight mass analyser, a quadrupole massanalyser, a Fourier Transform Ion Cyclotron Resonance (“FTICR”) massanalyser, a 2D (linear) quadrupole ion trap, a 3D (Paul) quadrupole iontrap or a magnetic sector mass analyser.

Preferably, in a mode of operation the ion guide may be maintained inuse at relatively high pressures, e.g. greater than or equal to 0.0001mbar, greater than or equal to 0.0005 mbar, greater than or equal to0.001 mbar, greater than or equal to 0.005 mbar, greater than or equalto 0.01 mbar, greater than or equal to 0.05 mbar, greater than or equalto 0.1 mbar, greater than or equal to 0.5 mbar, greater than or equal to1 mbar, greater than or equal to 5 mbar, greater than or equal to 10mbar, less than or equal to 10 mbar, less than or equal to 5 mbar, lessthan or equal to 1 mbar, less than or equal to 0.5 mbar, less than orequal to 0.1 mbar, less than or equal to 0.05 mbar, less than or equalto 0.01 mbar, less than or equal to 0.005 mbar, less than or equal to0.001 mbar, less than or equal to 0.0005 mbar, or less than or equal to0.0001 mbar. The ion guide may be maintained in use at a pressurebetween 0.0001 and 10 mbar, between 0.0001 and 1 mbar, between 0.0001and 0.1 mbar, between 0.0001 and 0.01 mbar, between 0.0001 and 0.001mbar, between 0.001 and 10 mbar, between 0.001 and 1 mbar, between 0.001and 0.1 mbar, between 0.001 and 0.01 mbar, between 0.01 and 10 mbar,between 0.01 and 1 mbar, between 0.01 and 0.1 mbar, between 0.1 and 10mbar, between 0.1 and 1 mbar, or between 1 and 10 mbar.

According to other embodiments the ion guide may be maintained in use atrelatively low pressures, e.g. greater than or equal to 1×10⁻⁷ mbar,greater than or equal to 5×10⁻⁷ mbar, greater than or equal to 1×10⁻⁶mbar, greater than or equal to 5×10⁻⁶ mbar, greater than or equal to1×10⁻⁵ mbar, and greater than or equal to 5×10⁻⁵ mbar, less than orequal to 1×10⁻⁴ mbar, less than or equal to 5×10⁻⁵ mbar, less than orequal to 1×10⁻⁵ mbar, less than or equal to 5×10⁻⁶ mbar, less than orequal to 1×10⁻⁶ mbar, less than or equal to 5×10⁻⁷ mbar, or less than orequal to 1×10⁻⁷ mbar. The ion guide may be maintained at a pressurebetween 1×10⁻⁷ and 1×10⁻⁴ mbar, between 1×10⁻⁷ and 5×10⁻⁵ mbar, between1×10⁻⁷ and 1×10⁻⁵ mbar, between 1×10⁻⁷ and 5×10⁻⁶ mbar, between 1×10⁻⁷and 1×10⁻⁶ mbar, between 1×10⁻⁷ and 5×10⁻⁷ mbar, between 5×10⁻⁷ and1×10⁻⁴ mbar, between 5×10⁻⁷ and 5×10⁻⁵ mbar, between 5×10⁻⁷ and 1×10⁻⁵mbar, between 5×10⁻⁷ and 5×10⁻⁶ mbar, between 5×10⁻⁷ and 1×10⁻⁶ mbar,between 1×10⁻⁶ mbar and 1×10⁻⁴ mbar, between 1×10⁻⁶ and 5×10⁻⁵ mbar,between 1×10⁻⁶ and 1×10⁻⁵ mbar, between 1×10⁻⁶ and 5×10⁻⁶ mbar, between5×10⁻⁶ mbar and 1×10⁻⁴ mbar, between 5×10⁻⁶ and 5×10⁻⁵ mbar, between5×10⁻⁶ and 1×10⁻⁵ mbar, between 1×10⁻⁵ mbar and 1×10⁻⁴ mbar, between1×10⁻⁵ and 5×10⁻⁵ mbar, or between 5×10⁻⁵ and 1×10⁻⁴ mbar.

From another aspect the present invention provides a mass spectrometercomprising an ion guide having a guide wire, cylindrical or rodelectrode and an outer cylindrical electrode wherein, in use, both an ACand a DC potential difference is maintained between the guide wire,cylindrical or rod electrode and the outer cylindrical electrode.

From another aspect the present invention provides a method of massspectrometry, comprising guiding ions along an ion guide comprising anouter electrode and an inner electrode disposed within the outerelectrode, maintaining the inner and outer electrodes at a DC potentialdifference such that ions experience a first radial force towards theinner electrode and applying an AC or RF voltage to the inner and/or theouter electrodes so that ions experience a second radial force towardsthe outer electrode.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst theions are being transported axially through the ion guide. Preferably,the guide wire comprises a semiconductor or resistive wire so that anaxial DC field is maintained, in use, along the ion guide by theapplication of a DC voltage between the ends of the guide wire.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in aplurality of outer concentric cylindrical electrodes wherein both AC andDC voltages are applied, in use, between the guide wire and theplurality of outer concentric cylindrical electrodes in order toradially retain ions whilst the ions are being transported axiallythrough the ion guide. Preferably, an axial DC field is maintained, inuse, along the ion guide by the application of DC voltages to theplurality of outer cylindrical electrodes. Travelling potential wavefunctions may be applied, in use, to the outer cylindrical electrodes toassist in ion transmission.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode. The ions are arranged, in use, to impact theinside wall of the cylindrical tube electrode or the guide wire toproduce secondary ion disassociation by adjusting the AC or DC voltages.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode. The AC voltage or the DC voltage is adjustedso as to cause an increase in the internal energy of ions within the ionguide thereby inducing collisional fragmentation or collisional induceddisassociation of the ions.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising an inner cylindrical electrode heldcentrally in an electrically conductive cylindrical tube electrodewherein both AC and DC voltages are applied, in use, between the innercylindrical electrode and the cylindrical tube electrode in order toradially retain ions whilst the ions are being transported axiallythrough the ion guide.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst theions are being transported axially through the ion guide and wherein theguide wire splits into two or more wires. In one embodiment different ACor DC voltages are applied to the two or more wires.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst theions are being transported axially through the ion guide and wherein theguide wire is not straight. In one embodiment the guide wire iscircular.

From another aspect the present invention provides a mass spectrometercomprising an ion guide, the ion guide comprising a Y-shaped outercylindrical electrode and a Y-shaped inner guide wire electrode. In use,the outer electrode and the inner electrode are supplied with both an ACvoltage and a DC voltage and the ion guide is arranged so that an ionbeam is split or ion beams are joined.

From another aspect the present invention provides a mass spectrometercomprising an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst theions are being transported axially through the ion guide. The ion guidefurther comprises a ring lens, plate or grid and an additional DC or ACvoltage is applied, in use, to the ring lens, plate or grid so that ionsare reflected backwards and are trapped or stored within the ion guide.

The ion guide according to the preferred embodiment has both DC andAC/RF voltages applied to the inner and/or outer electrodes. The DCpotential difference between the inner and outer electrodes causes ionsof one polarity to be attracted to the inner electrode as with aconventional guide wire ion guide. However, the AC/RF voltages appliedto one or both electrodes also generates a force which repels ions awayfrom the inner electrode, irrespective of the polarity of the ions. Theinhomogeneity of the AC/RF electric field between the electrodesincreases closer to the inner electrode. Ions of both polarities willdrift from regions of relatively high AC electric field inhomogeneity toregions of relatively low AC electric field inhomogeneity. Therefore,ions of both polarities will tend to drift away from the inner guidewire electrode and will move towards the outer cylindrical electrode.The AC/RF and DC voltages applied to the inner and/or outer electrodestherefore create a pseudo-potential well wherein the forces on ions of aparticular polarity are balanced in an annular region or channelarranged between the inner and outer electrodes.

The ion guide of the preferred embodiment is different from conventionalmultipole rod sets and stacked ring ion tunnel ion guides in which RFvoltages generate a pseudo-potential well which is aligned with thecentral axis of ion guide. Furthermore, the preferred ion guide issimpler and less expensive to manufacture than conventional multipolerod set ion guides and provides increased flexibility in the analysisand transmission of ions.

The preferred embodiment comprises an ion guide comprising a guide wireelectrode arranged centrally within an outer cylindrical electrode.AC/RF and DC voltages are preferably applied to both the guide wireand/or the outer cylindrical electrode to radially confine the ionswithin an annular region whilst they pass axially through the ion guide.Collisional gas may be present or introduced into the ion guide in orderto collisionally cool or alternatively to collisionally heat the ions.The voltages applied to the guide wire and the outer electrode and thediameters of the guide wire and the outer electrode determine whethercollisional cooling or heating occurs within the ion guide.

The potential V_(DC)(r) due to a DC potential difference V_(DC) beingmaintained between the guide wire inner electrode and the cylindricalouter electrode as a function of radius r from the guide wire innerelectrode is given as follows, where R_(wire) and R_(cylinder) are theradii of the guide wire and cylindrical outer electrode respectively:

${V_{DC}(r)} = {V_{DC}\left\lbrack \frac{\ln\left( \frac{r}{R_{cylinder}} \right)}{\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)} \right\rbrack}$

The potential difference due to the DC potentials applied to the guidewire and outer electrode generate an electric field E_(DC)(r). Theelectric field strength E_(DC)(r) between the guide wire and thecylindrical electrode increases in a direction towards the guide wireand is given below as a function of the radius r from the wire:

${E_{DC}(r)} = \frac{V_{DC}}{r \cdot {\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)}}$

Providing the ions are adiabatic and are moving relatively slowly in aninhomogeneous oscillatory electric field, the ion motion may beapproximated by a fast oscillating motion, synchronous with the AC/RFelectric field and superimposed on a slow drift motion. The drift motionis caused by the inhomogeneity of the electric fields and may beconsidered as if the ion is moving in an electrostatic potential orpseudo-potential.

The electric field due to the AC/RF voltages applied to the guide wireand outer electrode E_(RF)(r) at one instance in time as a function ofradius from the guide wire is given by:

${E_{RF}(r)} = \frac{V_{RF}}{r\; \cdot {\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)}}$

The radial AC/RF electric field E_(RF)(r,t) as a function of radius fromthe guide wire and time t may be given by the following equation, whereω is the angular frequency of the AC/RF radial electric field:E _(RF)(r,t)=E ^(RF)(r)cos(ωt)

The pseudo-potential energy P_(RF)(r) as a function of radius from theguide wire is given as follows, where q and m are the electronic chargeand mass of the ion respectively:

${P_{RF}(r)} = {q^{2}\frac{{E_{RF}(r)}^{2}}{4m\;\omega^{2}}}$

The combined effective potential V_(EFF)(r) as a function of radius fromthe guide wire is given by the pseudo-potential energy P_(RF)(r) dividedby the ion electric charge q summed with the potential due to the DCvoltages V_(DC)(r) applied to the guide wire and cylindrical electrode.Substituting the equation for E_(RF)(r) and the term for the DCpotential V_(DC)(r) from above gives the following combined effectivepotential V_(EFF)(r):

${V_{EFF}(r)} = {{q\frac{V_{RF}^{2}}{4{m\left( {r\;\omega\;{\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)}} \right)}^{2}}} + {V_{DC}\frac{\ln\left( \frac{r}{R_{cylinder}} \right)}{\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)}}}$

The pseudo-potential well approximation requires that the ion motion issuch that the ions are adiabatic. If the ions are not adiabatic thenthey will gain kinetic energy from the oscillatory electric field andwill be ejected from the ion guide. An adiabaticity parameter Adiab(r)for radial fields with no axial components is given by:

${{Adiab}(r)} = \frac{2{{q\left( {\frac{\mathbb{d}}{\mathbb{d}r}{E_{RF}(r)}} \right)}}}{m\;\omega^{2}}$

Substituting the equation for the AC/RF radial electric field E_(RF)(r)into the equation for the adiabaticity parameter gives:

${{Adiab}(r)} = {2{{q\frac{V_{RF}}{r^{2}m\;\omega^{2}{\ln\left( \frac{R_{wire}}{R_{cylinder}} \right)}}}}}$

Empirically, provided ions are relatively slow and the adiabaticityparameter is below 0.4 then the pseudo-potential approximation holds.

DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, byway of example only, and with reference to the accompanying drawings inwhich:

FIG. 1A shows a conventional quadrupole rod set ion guide wherein ACvoltages of opposite phases are supplied to adjacent rods, FIG. 1B showsa conventional ion tunnel ion guide wherein AC voltages of oppositephase are supplied to alternate rings and FIG. 1C shows a conventionalguide wire ion guide comprising a guide wire arranged along the centralaxis of a cylindrical tube electrode wherein a DC potential differenceis maintained between the guide wire and the outer cylindricalelectrode;

FIG. 2A shows a schematic of a guide wire ion guide according to thepreferred embodiment comprising an outer cylindrical conductingelectrode and an inner guide wire electrode arranged along the centralaxis of the cylindrical electrode wherein a DC potential difference ismaintained between the guide wire and cylindrical electrodes and an ACor RF voltage is applied to the cylindrical electrode and/or the guidewire, and FIG. 2B shows a schematic of an ion guide according to afurther preferred embodiment wherein the outer cylindrical electrode issegmented;

FIG. 3 shows the potential profile in the region between the guide wireand the outer cylindrical electrode when only DC voltages are applied tothe cylindrical electrode and the guide wire;

FIG. 4 shows the adiabaticity parameter in the region between the guidewire and the outer cylindrical electrode for ions having a mass tocharge ratio of 1000;

FIG. 5 shows the pseudo-potential profile in the region between theguide wire and the outer cylindrical electrode for ions having a mass tocharge ratio of 1000 when both DC voltages and AC/RF voltages areapplied to the cylindrical electrode and guide wire;

FIG. 6 shows the pseudo-potential profile in the region between theguide wire and the outer cylindrical electrode for ions having a mass tocharge ratio of 1000 and 2000 when both DC and AC/RF voltages areapplied to the cylindrical electrode and the guide wire;

FIG. 7 shows an ion simulation illustrating the ion motion in a guidewire ion guide for three ions having identical mass to charge ratios of1000, initial kinetic energies of 8 eV and being released at a distanceof 1.45 mm from the central axis and at angles of 45°, 0° and −45°relative to the guide wire;

FIG. 8 shows an ion simulation illustrating the ion motion in a guidewire ion guide for three ions having identical mass to charge ratios of1000, less energetic initial kinetic energies of 4 eV and being releasedat a distance of 1.45 mm from the central axis and at angles of 45°, 0°and −45° relative to the guide wire;

FIG. 9 shows an ion simulation illustrating the ion motion in a guidewire ion guide for three ions having identical mass to charge ratios of3000, initial kinetic energies of 4 eV and being released at a distanceof 1.45 mm from the central axis and at angles of 45°, 0° and −45°relative to the guide wire; and

FIG. 10 shows an ion simulation illustrating the ion motion in a guidewire ion guide for ions having identical mass to charge ratios of 1000both with and without the presence of nitrogen gas at a pressure of 1mbar wherein the ions have initial kinetic energies of 8 eV and arereleased at a distance of 1.45 mm from the central axis and at an angleof 45° relative to the guide wire.

DETAILED DESCRIPTION OF THE DRAWINGS

The differences between a guide wire ion guide according to thepreferred embodiment and other conventional ion guides will beillustrated by referring to some conventional forms of ion guide shownin FIGS. 1A-1C. FIG. 1A shows a conventional quadrupole rod set ionguide comprising a set of parallel rod electrodes. In this arrangementAC/RF voltages of opposite phases are supplied to adjacent rods so thatinhomogeneous AC/RF electric fields generate a pseudo-potential wellalong the central axis of the rod set. Ions are confined within thispseudo-potential well and may be guided through the quadrupole rod set.FIG. 1B shows an ion tunnel ion guide comprising a stacked concentriccircular ring set of electrodes wherein ions are transmitted through theapertures in the ring electrodes. The apertures are typicallysubstantially all the same size. In this arrangement AC/RF voltages ofopposite phases are supplied to alternate rings of the ion tunnel ionguide to generate a pseudo-potential well along the central axis of theion guide which acts to radially confine ions which are passed throughthe ion guide. FIG. 1C shows a conventional guide wire ion guidecomprising a guide wire electrode arranged along the central axis of acylindrical tube electrode. In this arrangement a negative DC voltage issupplied to the guide wire to attract positive ions and a positive DCvoltage is supplied to the outer cylindrical electrode to repel positiveions. Ions which enter the guide wire ion guide will follow ellipticalpaths around the guide wire under conditions of high vacuum.Conventional guide wire ion guides as shown in FIG. 1C are thereforeonly used to transport ions in regions of relatively low pressurewherein ion collisions with gas molecules are unlikely, otherwise thevelocity of the ions would be dampened and the ions would discharge uponhitting the central guide wire with the result that the transmissionefficiency would be near zero.

FIG. 2A shows a preferred embodiment of the present invention comprisinga guide wire ion guide 1 comprising an outer cylindrical conductingelectrode 2 and an inner guide wire electrode 3. According to apreferred embodiment the outer electrode 2 and the guide wire electrode3 are coaxial. In operation DC voltages V_(DC) are applied to the outerelectrode 2 and/or the inner guide wire 3 so that a DC potentialdifference is maintained between the outer electrode 2 and the guidewire 3 in order to attract ions of one polarity towards the guide wire3. AC or RF voltages V_(RF) are also applied to the outer electrode 2and/or the guide wire 3 so that ions irrespective of their polarity willbe forced radially outwards by the inhomogeneous AC electric field. FIG.2B shows a further preferred embodiment wherein the ion guide 1comprises a stacked ring set outer electrode 2 wherein the outerelectrode comprises a plurality of concentric cylindrical electrodes 2.In this embodiment the inner guide wire electrode 3 is arranged alongthe central axis of the stacked ring set 2. In operation AC/RF and DCvoltages are supplied to the guide wire 3 and at least some of thecylindrical electrodes forming the outer electrode 2. In a preferredembodiment different AC/RF and/or DC voltages are applied to at leastsome of the cylindrical electrodes 2. An axial DC electric field maytherefore be created by maintaining DC potential differences between thecylindrical electrodes 2 such that an axial DC voltage gradient ismaintained along at least a portion of the guide wire ion guide 1. Theaxial DC voltage gradient may be used to urge ions along at least aportion of the ion guide 1 or to constrain the ions axially. Accordingto a further embodiment travelling or transient DC potential waveformsor DC voltages may be applied to the ion guide 1 by varying the DCvoltages applied to the cylindrical electrodes 2 with time. Thetransient DC voltages or waveforms may move along at least a portion ofthe ion guide 1 to urge ions along the ion guide 1. The transient DCvoltages or waveforms may have amplitudes, wavelengths or frequencieswhich remain constant or vary with time. The transient DC voltages orwaveforms may also be generated repeatedly at a frequency which eitherremains constant or varies with time. In one embodiment two or moretransient DC voltages or waveforms pass simultaneously along the ionguide.

In a further embodiment the mass spectrometer may comprise componentslocated downstream of the ion guide 1 whose operation is synchronisedwith the pulses of ions emerging from the ion guide. For example, an iondetector, pusher electrode of a Time of Flight mass analyser, ion trapor mass filter may be substantially synchronised with the pulses of ionsemerging from the ion guide 1 when transient DC voltages are applied tothe ion guide 1.

According to the preferred embodiment DC and AC/RF voltages are suppliedto both the outer electrode 2 and inner electrode 3. However, accordingto other embodiments the AC/RF and/or DC voltages may only be applied toeither the outer electrode 2 or the inner electrode 3, i.e. not both.

According to a less preferred embodiment the inner electrode may bedisplaced radially from the central axis of the outer electrode 2.

FIG. 3 shows the potential profile between the guide wire 3 and theouter cylindrical electrode 2 when only DC voltages are applied to thetwo electrodes 2, 3. The outer electrode 2 had a radius of 5 mm and wasgrounded and the guide wire 3 had a radius of 0.025 mm and wasmaintained at −10 V. The application of DC voltages to the outerelectrode 2 and the guide wire 3 generated a steep logarithmic potentialwell centred on the guide wire 3. It is apparent that ions will eitherbe attracted to or repelled from the guide wire 3 depending upon thepolarity of the ions. By supplying the outer electrode 2 and the guidewire 3 with AC/RF voltages according to the preferred embodiment, theradial force attracting ions to the guide wire 3 can becounter-balanced. The electric field inhomogeneities due to the AC/RFpotentials force ions of both polarities radially outward. Therefore, byappropriate selection of the DC and AC/RF voltages which are applied toboth the guide wire 3 and/or the outer electrode 2, the inward and theoutward radial forces can be balanced for at least some of the ionsbeing transmitted through the ion guide 1. Ions are therefore preferablyconfined in a pseudo-potential well within an annulus between the guidewire 3 and the outer electrode 2.

The pseudo-potential approximation requires that the ion motion is suchthat the ions are adiabatic. If the ions are not adiabatic then theywill gain kinetic energy from the oscillatory AC/RF electric fields andwill hence be ejected from the ion guide 1. The ions adiabaticity can bedetermined by an adiabaticity parameter which varies according to themass to charge ratio of the ion, the distance of the ion from the guidewire 3, the dimensions of the ion guide 1 and the AC/RF electric fieldparameters. If ions have an adiabaticity parameter which is sufficientlylow then they can be said to be adiabatic and hence will remain stablewithin the ion guide 1.

FIG. 4 shows the adiabaticity parameter in the region between the guidewire 3 and the outer cylindrical electrode 2 as a function of radiusfrom the guide wire 3 for ions having a mass to charge ratio of 1000. Inthis example the cylindrical electrode 2 was grounded and the guide wire3 was maintained at −30 V to create a DC potential difference of −30 V.The outer electrode 2 and the guide wire 3 were connected to an RFvoltage supply of 900 V having a frequency of 11 rad/μs (AC frequency of1.75 MHz). As the ions approach the guide wire 3 (i.e. as the radiusdecreases) the adiabaticity parameter of the ions increases and the ionsbegin to pick up energy from the oscillating AC/RF electric field. Ifthe adiabaticity parameter increases above a threshold value (e.g. about0.4) then the ions will pick up an excessive amount of kinetic energyand will no longer be stable in the pseudo-potential well. Therefore, ifions travel too close to the guide wire 3 then they may not betransmitted by the ion guide 1.

The potential between the guide wire 3 and the outer electrode 2 due tothe DC voltages applied to them is independent of the ion mass m andcharge q. However, the potential due to the AC/RF voltages isproportional to the mass to charge ratio of the ion (q/m). Hence, theposition and magnitude of the pseudo-potential well is a function of themass to charge ratio of the ions.

FIG. 5 illustrates the pseudo-potential profile in the region betweenthe guide wire 3 and the outer cylindrical electrode 2 for ions having amass to charge ratio of 1000 when both DC and AC/RF voltages are appliedto the electrodes 2, 3. In this example the guide wire 3 has a radius of0.025 mm and the outer electrode 2 has a radius of 5 mm. The cylindricalelectrode 2 is grounded and the guide wire 3 is maintained at −30 V tocreate a DC potential difference of −30 V. The outer electrode 2 and theguide wire 3 are also connected to an RF voltage supply of 900 V havinga frequency of 11 rad/μs (AC frequency of 1.75 MHz). The combination ofDC and AC voltages provides a pseudo-potential well in an annulusbetween the guide wire 3 and outer electrode 2 which is centredapproximately 1.4 mm radially outward from the central guide wire 3.Accordingly, provided ions enter the guide wire ion guide 1 relativelyslowly and have a suitably low adiabaticity parameter then they willremain confined within the potential well and will be transmittedthrough the ion guide 1.

FIG. 6 shows the pseudo-potential profile in the region between theguide wire 3 and the outer cylindrical electrode 2 for ions having massto charge ratios of 1000 and 2000 when both DC and AC/RF voltages areapplied to the electrodes 2, 3. The guide wire 3 has a radius of 0.025mm and the outer electrode has a radius of 5 mm. The cylindricalelectrode 2 was grounded and the guide wire 3 was maintained at −30 V tocreate a DC potential difference of −30 V. The outer electrode 2 and theguide wire 3 were also connected to an RF voltage supply of 900 V havinga frequency of 11 rad/μs (AC frequency of 1.75 MHz). Thepseudo-potential profile for ions having a mass to charge ratio of 1000is shown by the solid line and the pseudo-potential profile for ionshaving a higher mass to charge ratio of 2000 is shown by the dashedline. It can be seen that ions having a mass to charge ratio of 1000have a pseudo-potential well centred at a radius approximately 1.4 mmfrom the guide wire 3, whereas ions having a mass to charge ratio of2000 have a deeper pseudo-potential well centred at a radiusapproximately 0.9 mm from the guide wire 3, i.e. closer to the guidewire 3.

In a preferred embodiment a gas is either present in or is introducedinto the guide wire ion guide 1. Ions may be cooled by repetitivecollisions with the gas molecules such that the ions will tend tocongregate near the bottom of their respective pseudo-potential wells.Accordingly, ions having lower mass to charge ratios will congregate inannular regions at larger radii from the guide wire 3 whereas ionshaving relatively higher mass to charge ratios will congregate inannular regions closer to the guide wire 3. Therefore, ions having lowermass to charge ratios will orbit the guide wire 3 at larger radii thanions having relatively higher mass to charge ratios. As such, the ionguide 1 may be used according to a less preferred embodiment to separateions according to their mass to charge ratios. In one embodiment theAC/RF and/or DC voltages applied to the outer electrode 2 and to theguide wire 3 may be varied or scanned such that ions having a desiredrange of mass to charge ratios are arranged to congregate at either theguide wire 3 or the outer electrode 2 and hence will be lost from theion guide 1. Ions may therefore be filtered according to their mass tocharge ratio.

According to another embodiment the AC/RF and/or DC voltages applied tothe electrodes forming the ion guide 1 may be arranged such that theions are caused to increase in internal energy so that collisionalfragmentation or Collisional Induced Disassociation (“CID”) results.According to another embodiment the AC/RF and/or DC voltages applied tothe ion guide 1 may be arranged such that ions impact either the outerelectrode 2 or the guide wire 3 to induce Secondary Ion Disassociation(SID).

Ion motion through a guide wire ion guide 1 according to the preferredembodiment was simulated using a SIMION numerical ion simulation program(version 7.0). The resulting simulations are shown in FIGS. 7-10.

FIG. 7 shows a simulation for the ion motion through a preferred ionguide 1 for three ions 4, 5, 6 having a mass to charge ratio of 1000,initial kinetic energies of 8 eV, being released at a distance of 1.45mm from the central axis and at an angle of 45°, 0° and −45° relative tothe guide wire 3. The cylindrical electrode 2 and the guide wire 3 weremaintained at 0 V DC and −30 V DC respectively. The outer electrode 2and the guide wire 3 are also connected to an RF voltage supply of 900 Vhaving a frequency of 11 rad/μs (AC frequency of 1.75 MHz). In thissimulation the ions 4, 5, 6 were released at the entrance 9 to thepreferred ion guide 1 at a radius from the guide wire 3 which wasapproximately at the centre of the pseudo-potential well. The ions 4which entered the ion guide 1 at an angle of 0° relative to the guidewire 3 passed from the entrance 9 of the ion guide 1 to the exit 10along a path which was substantially parallel to the guide wire 3. Theseions 4 remained stable in the pseudo-potential well and were radiallyconfined and transmitted through the ion guide 1.

Ions 5 which entered the ion guide 1 at an angle of 45° relative to theguide wire 3, traveled radially outward towards the outer electrode 2away from the centre of the pseudo-potential well until they wereattracted back towards the guide wire 3 by the force due to the appliedDC voltages. The ions 5 then traveled towards the guide wire 3 and pastthe centre of the pseudo-potential well until the force due to the AC/RFfields repelled them back towards the outer electrode 2. In this mannerthe ions 5 oscillate radially in the pseudo-potential well whilst theypass along the ion guide 1. However, as the ions 5 oscillate they travelto a radius which is relatively close to the guide wire 3 and at whichthe radial electric field gradient is high. At such a small radius fromthe guide wire 3 the adiabaticity parameter of the ions 5 increases andthe ions 5 can no longer be said to be adiabatic. The ions thereforepick up kinetic energy from the oscillating AC/RF electric fields andare repelled from the guide wire 3 with excessive radial energy suchthat they ultimately strike the outer electrode 2. The ions 5 whichstrike the outer electrode 2 are neutralised and are not transmitted bythe ion guide 1. Therefore, the AC/RF and/or DC voltages may be selectedsuch that ions which enter the ion guide 1 at certain angles relative tothe guide wire 3 are not transmitted.

Ions 6 which entered the ion guide 1 at an angle of −45° with respect tothe guide wire 3 also oscillated radially in the pseudo-potential wellas they traveled axially. Although the ions 6 do pass close to the guidewire 3 and pick up a slight amount of radial kinetic energy the acquiredkinetic energy is not excessive and as such the ions 6 do not strike theouter electrode 2. Accordingly, the ions 6 oscillate radially in thepseudo-potential well and are transmitted from the entrance 9 to theexit 10 of the ion guide 1.

FIG. 8 shows a simulation for the ion motion through a preferred ionguide 1 for three ions 4, 5, 6 having mass to charge ratios of 1000,initial kinetic energies of 4 eV and being released at a distance of1.45 mm from the central axis and at an angle of 45°, 0° and −45°relative to the guide wire 3. The outer cylindrical electrode 2 and theguide wire 3 were maintained at 0 V DC and −30 V DC respectively. Theouter electrode 2 and the guide wire 3 are also connected to an RFvoltage supply of 900 V having a frequency of 11 rad/μs (AC frequency of1.75 MHz). In this simulation the ions 4, 5, 6 have half of the initialkinetic energy of the ions shown and described in relation to FIG. 7.All the ions 4, 5, 6 remain at radii from the guide wire 3 wherein theadiabaticity parameter is below the threshold at which ions 4, 5, 6could gain a substantial amount of radial kinetic energy from the AC/RFelectric fields. As such, all the ions 4, 5, 6 remain radially confinedwithin and are transmitted through the ion guide 1 irrespective ofwhether their entrance angle is 45°, 0° or −45° with respect to theguide wire 3.

FIG. 9 shows a simulation for the ion motion through a preferred ionguide 1 for three ions 4, 5, 6 having a mass to charge ratios of 3000,initial kinetic energies of 4 eV and being released at a distance of1.45 mm from the central axis and at an angle of 45°, 0° and −45°relative to the guide wire 3. The outer cylindrical electrode 2 and theguide wire 3 were maintained at 0 V DC and −30 V DC respectively. Theouter electrode 2 and the guide wire 3 are also connected to an RFvoltage supply of 900 V having a frequency of 11 rad/μs (AC frequency of1.75 MHz). In this simulation the ions have a higher mass to chargeratio than the ions shown and described in relation to FIG. 8 andtherefore have a pseudo-potential well which is deeper and centred at aradius closer to the guide wire 3. Ions 4 which enter the ion guide 1 atan angle of 0° relative to the guide wire 3 and at a position which isradially outward from the centre of the pseudo-potential well oscillateabout the centre of the well as they are transmitted from the entrance 9to the exit 10 of the ion guide 1. Ions 5 which enter the ion guide 1 at45° relative to the guide wire 3 also oscillate about the centre of thewell as they are transmitted to the exit 10. Ions 6 which enter the ionguide at −45° relative to the guide wire 3 have an initial radialvelocity towards the guide wire 3 and travel closer to the guide wire 3than the other ions 4, 5. The ions 6 therefore reach radii at which theions 6 have a higher adiabaticity parameter and pick up some kineticenergy from the AC/RF electric field. However, the ions 6 do not pick upsufficient energy to become unstable in the ion guide 1 and hence do nothit the outer electrode 2. Accordingly, all the ions 4, 5, 6 aretransmitted to the exit 10 of the ion guide 1.

FIG. 10 shows a simulation for the ion motion through an ion guide 1 fortwo ions 7, 8 having a mass to charge ratio of 1000, initial kineticenergies of 8 eV and wherein the ions are released at a distance of 1.45mm from the central axis and at an angle of 45° relative to the guidewire 3. The outer cylindrical electrode 2 and guide wire 3 aremaintained at 0 V and −30 V DC respectively. The outer electrode 2 andthe guide wire 3 are also connected to an RF voltage supply of 900 Vhaving a frequency of 11 rad/μs (AC frequency of 1.75 MHz). In thissimulation an additional axial electric field of 0.1 V/mm was maintainedalong the length of the ion guide 1.

Ions 7 which enter the ion guide 1 at an angle of 45° relative to theguide wire 3 when no gas is present in the ion guide 1 travel to aradius which is relatively close to the guide wire 3 and pick up radialkinetic energy from the AC/RF electric field. This extra kinetic energyeventually causes the ions 7 to collide with the outer electrode 2 suchthat they are neutralised and not transmitted by the ion guide 1.

If a cooling gas is present or introduced into the ion guide 1, then asshown in FIG. 10, the ions 8 take a quite different path through the ionguide 1. FIG. 10 shows a simulation of the path of ions 8 through theion guide 1 when nitrogen gas is present at a pressure of 1 mbar.Collisions between the ions 8 and the gas molecules help to reduce thekinetic energy imparted to the ions 8 when they travel relatively closeto the guide wire 3. Therefore, the presence of the cooling gas preventsthe ions 8 from gaining excessive radial kinetic from the AC/RF electricfields and as such the ions 8 are prevented from becoming unstable andleaving the pseudo-potential well.

The gas introduced into the ion guide 1 may eventually reduce the axialenergy of the ions to the thermal energy of the gas. Therefore, anadditional axial electric field may be applied to maintain ion motion inthe axial direction. The axial electric field may be achieved bydividing the outer electrode 2 into a series of concentric cylindricalelectrodes and maintaining DC potential differences between thecylindrical electrodes such that an axial DC voltage gradient ismaintained over at least a portion of the length of the ion guide 1. Ina further embodiment, travelling potential wave functions may be appliedto the elements of the outer segmented electrode 2 in order to assist inion transmission through the ion guide 1.

In one embodiment the guide wire 3 may comprise a semiconductor orresistive wire such that an axial DC electric field may be generatedwhen a DC potential difference is maintained across the guide wire 3.The guide wire 3 may also be formed of two or more sections, eachsection having different AC/RF and/or DC voltages applied thereto.

The ion guide 1 may be formed in any shape. For example, the ion guide 1may be bent in a circle or other shape to guide ions around corners. Inan embodiment the guide wire 3 and/or outer electrode(s) 2 may beY-shaped or otherwise arranged so as to split or join packets or beamsof ions.

Although the outer electrode 2 and inner electrode 3 have been describedaccording to the preferred embodiment as being cylindrical electrodesand wires it is also contemplated that according to less preferredembodiments the outer electrode may comprise a rod set or segmented rodset and/or the inner electrode may comprise a cylindrical or rodelectrode.

In another embodiment the entrance 9 and/or exit 10 of the ion guide 1may be arranged at a higher or lower potential so that ions approachingthe entrance 9 and/or exit 10 of the ion guide 1 are reflected and maybe trapped or stored within the ion guide 1. These regions of higher orlower potential may be generated by additional DC and/or AC/RF voltagesbeing applied to one or more ring lenses, plates or grids arrangedsubstantially at the entrance 9 and/or exit 10 of the ion guide 1.

Although the present invention has been described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes in form and detail may be made without departingfrom the scope of the invention as set forth in the accompanying claims.

1. A mass spectrometer comprising an ion guide, said ion guidecomprising an outer electrode and an inner electrode disposed withinsaid outer electrode, wherein in use said inner and outer electrodes aremaintained at a DC potential difference such that ions experience afirst radial force towards said inner electrode and wherein in use an ACor RF voltage is applied to said inner and/or said outer electrodes sothat ions experience a second radial force towards said outer electrode.2. A mass spectrometer as claimed in claim 1, wherein said AC or RFvoltage is a single phase AC or RF voltage applied to said innerelectrode.
 3. A mass spectrometer as claimed in claim 1, wherein said ACor RF voltage is a single phase AC or RF voltage applied to said outerelectrode.
 4. A mass spectrometer as claimed in claim 1, wherein said ACor RF voltage is a two phase AC or RF voltage and wherein a first phaseis applied to said inner electrode and a second phase is applied to saidouter electrode.
 5. A mass spectrometer as claimed in claim 1, whereinsaid AC or RF voltage has a frequency selected from the group consistingof: (i)<100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv) 300-400 kHz;(v) 400-500 kHz; (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz; (viii) 1.5-2.0MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz; (xii) 3.5-4.0MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5 MHz; (xvi)5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix) 7.0-7.5 MHz;(xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxii) 8.5-9.0 MHz; (xxiii) 9.0-9.5MHz; (xxiv) 9.5-10.0 MHz; and (xxv)>10.0 MHz.
 6. A mass spectrometer asclaimed in claim 1, wherein the amplitude of said AC or RF voltage isselected from the group consisting of: (i)<50 V peak to peak; (ii)50-100 V peak to peak; (iii) 100-150 V peak to peak; (iv) 150-200 V peakto peak; (v) 200-300 V peak to peak; (vi) 300-400 V peak to peak; (vii)400-500 V peak to peak; (viii) 500-600 V peak to peak; (ix) 600-700 Vpeak to peak; (x) 700-800 V peak to peak; (xi) 800-900 V peak to peak;(xii) 900-1000 V peak to peak; (xiii) 1000-1100 V peak to peak; (xiv)1100-1200 V peak to peak; (xv) 1200-1300 V peak to peak; (xvi) 1300-1400V peak to peak; (xvii) 1400-1500 V peak to peak; and (xviii)>1500 V peakto peak.
 7. A mass spectrometer as claimed in claim 1, wherein saidouter electrode is maintained, in use, at a DC potential selected fromthe group consisting of: (i)<−500 V; (ii) −500 to −400 V; (iii) −400 to−300 V; (iv) −300 to −200 V; (v) −200 to −100 V; (vi) −100 to −75 V;(vii) −75 to −50 V; (viii) −50 to −25 V; (ix) −25 to 0V; (x) 0V; (xi)0-25 V; (xii) 25-50 V; (xiii) 50-75 V; (xiv) 75-100 V; (xv) 100-200 V;(xvi) 200-300 V; (xvii) 300-400 V; (xviii) 400-500 V; (xix)>500 V.
 8. Amass spectrometer as claimed in claim 1, wherein said inner electrode ismaintained, in use, at a DC potential selected from the group consistingof: (i) <−500 V; (ii) −500 to −400 V; (iii) −400 to −300 V; (iv) −300 to−200 V; (v) −200 to −100 V; (vi) −100 to −75 V; (vii) −75 to −50 V;(viii) −50 to −25 V; (ix) −25 to 0V; (x) 0V; (xi) 0-25 V; (xii) 25-50 V;(xiii) 50-75 V; (xiv) 75-100 V; (xv) 100-200 V; (xvi) 200-300 V; (xvii)300-400 V; (xviii) 400-500 V; (xix)>500 V.
 9. A mass spectrometer asclaimed in claim 1, wherein said outer electrode is maintained at a DCpotential which is more positive than the DC potential at which saidinner electrode is maintained, in use, by a potential differenceselected from the group consisting of: (i) 0.1-5 V; (ii) 5-10 V; (iii)10-15 V; (iv) 15-20 V; (v) 20-25 V; (vi) 25-30 V; (vii) 30-40 V; (viii)40-50 V; and (ix)>50 V.
 10. A mass spectrometer as claimed in claim 1,wherein said outer electrode is maintained at a DC potential which ismore negative than the DC potential at which said inner electrode ismaintained, in use, by a potential difference selected from the groupconsisting of: (i) 0.1-5 V; (ii) 5-10 V; (iii) 10-15 V; (iv) 15-20 V;(v) 20-25 V; (vi) 25-30 V; (vii) 30-40 V; (viii) 40-50 V; and (ix)>50 V.11. A mass spectrometer as claimed in claim 1, wherein said innerelectrode comprises a guide wire.
 12. A mass spectrometer as claimed inclaim 1, wherein at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%,95% or 100% of said inner electrode comprises a semiconductor orresistive wire and wherein, in use, an axial DC potential gradient ismaintained along at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%,95% or 100% of said inner electrode by applying a DC potentialdifference across 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or100% of said inner electrode.
 13. A mass spectrometer as claimed inclaim 1, wherein said inner electrode comprises a cylindrical electrode.14. A mass spectrometer as claimed in claim 13, wherein said innerelectrode comprises a plurality of concentric cylindrical electrodes.15. A mass spectrometer as claimed in claim 14, wherein, in use, anaxial DC potential gradient is maintained along at least 10%, 20%, 30%,40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of said inner electrode bymaintaining at least some of said plurality of concentric cylindricalelectrodes at different DC potentials.
 16. A mass spectrometer asclaimed in claim 1, wherein said inner electrode comprises a pluralityof electrodes.
 17. A mass spectrometer as claimed in claim 16, whereinin a mode of operation an axial DC potential gradient is maintainedalong at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100%of the length of said inner electrode so that ions are urged along atleast a portion of said ion guide.
 18. A mass spectrometer as claimed inclaim 17, wherein said axial DC potential gradient is maintainedsubstantially constant with time as ions pass along said ion guide. 19.A mass spectrometer as claimed in claim 17, wherein said axial DCpotential gradient varies with time as ions pass along said ion guide.20. A mass spectrometer as claimed in claim 1, wherein said outerelectrode comprises a plurality of electrodes.
 21. A mass spectrometeras claimed in claim 20, wherein in a mode of operation an axial DCpotential gradient is maintained along at least 10%, 20%, 30%, 40%, 50%,60%, 70%, 80%, 90%, 95% or 100% of the length of said outer electrode sothat ions are urged along at least a portion of said ion guide.
 22. Amass spectrometer as claimed in claim 21, wherein said axial DCpotential gradient is maintained substantially constant with time asions pass along said ion guide.
 23. A mass spectrometer as claimed inclaim 21, wherein said axial DC potential gradient varies with time asions pass along said ion guide.
 24. A mass spectrometer as claimed inclaim 1, wherein said ion guide comprises 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,30 or >30 segments, wherein each segment comprises 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,26, 27, 28, 29, 30 or >30 electrodes and wherein the electrodes in asegment are maintained at substantially the same DC potential.
 25. Amass spectrometer as claimed in claim 24, wherein a plurality ofsegments are maintained at substantially the same DC potential.
 26. Amass spectrometer as claimed in claim 24, wherein each segment ismaintained at substantially the same DC potential as the subsequent nthsegment wherein n is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or >30.
 27. Amass spectrometer as claimed in claim 1, wherein ions are constrainedaxially within said ion guide by a real potential barrier or well.
 28. Amass spectrometer as claimed in claim 1, wherein the transit time ofions through said ion guide is selected from the group consisting of:(i) less than or equal to 20 ms; (ii) less than or equal to 10 ms; (iii)less than or equal to 5 ms; (iv) less than or equal to 1 ms; and (v)less than or equal to 0.5 ms.
 29. A mass spectrometer as claimed inclaim 1, wherein in use one or more transient DC voltages or one or moretransient DC voltage waveforms are initially provided at a first axialposition and are then subsequently provided at second, then thirddifferent axial positions along said ion guide.
 30. A mass spectrometeras claimed in claim 1, wherein in use one or more transient DC voltagesor one or more transient DC voltage waveforms move in use from one endof said ion guide to another end of said ion guide so that ions areurged along said ion guide.
 31. A mass spectrometer as claimed in claim29, wherein said one or more transient DC voltages create: (i) apotential hill or barrier; (ii) a potential well; (iii) multiplepotential hills or barriers; (iv) multiple potential wells; (v) acombination of a potential hill or barrier and a potential well; or (vi)a combination of multiple potential hills or barriers and multiplepotential wells.
 32. A mass spectrometer as claimed in claim 29, whereinsaid one or more transient DC voltage waveforms comprise a repeatingwaveform.
 33. A mass spectrometer as claimed in claim 32, wherein saidone or more transient DC voltage waveforms comprise a square wave.
 34. Amass spectrometer as claimed in claim 29, wherein the amplitude of saidone or more transient DC voltages or said one or more transient DCvoltage waveforms remains substantially constant with time.
 35. A massspectrometer as claimed in claim 29, wherein the amplitude of said oneor more transient DC voltages or said one or more transient DC voltagewaveforms varies with time.
 36. A mass spectrometer as claimed in claim35, wherein the amplitude of said one or more transient DC voltages orsaid one or more transient DC voltage waveforms either: (i) increaseswith time; (ii) increases then decreases with time; (iii) decreases withtime; or (iv) decreases then increases with time.
 37. A massspectrometer as claimed in claim 29, wherein said ion guide comprises anupstream entrance region, a downstream exit region and an intermediateregion, wherein: in said entrance region the amplitude of said one ormore transient DC voltages or said one or more transient DC voltagewaveforms has a first amplitude; in said intermediate region theamplitude of said one or more transient DC voltages or said one or moretransient DC voltage waveforms has a second amplitude; and in said exitregion the amplitude of said one or more transient DC voltages or saidone or more transient DC voltage waveforms has a third amplitude.
 38. Amass spectrometer as claimed in claim 37, wherein the entrance and/orexit region comprise a proportion of the total axial length of said ionguide selected from the group consisting of: (i)<5%; (ii) 5-10%; (iii)10-15%; (iv) 15-20%; (v) 20-25%; (vi) 25-30%; (vii) 30-35%; (viii)35-40%; and (ix) 40-45%.
 39. A mass spectrometer as claimed in claim 37,wherein said first and/or third amplitudes are substantially zero andsaid second amplitude is substantially non-zero.
 40. A mass spectrometeras claimed in claim 37, wherein said second amplitude is larger thansaid first amplitude and/or said second amplitude is larger than saidthird amplitude.
 41. A mass spectrometer as claimed in claim 1, whereinone or more transient DC voltages or one or more transient DC voltagewaveforms pass in use along said ion guide with a first velocity.
 42. Amass spectrometer as claimed in claim 41, wherein said first velocity:(i) remains substantially constant; (ii) varies; (iii) increases; (iv)increases then decreases; (v) decreases; (vi) decreases then increases;(vii) reduces to substantially zero; (viii) reverses direction; or (ix)reduces to substantially zero and then reverses direction.
 43. A massspectrometer as claimed in claim 41, wherein said one or more transientDC voltages or said one or more transient DC voltage waveforms causesions within said ion guide to pass along said ion guide with a secondvelocity.
 44. A mass spectrometer as claimed in claim 43, wherein thedifference between said first velocity and said second velocity is lessthan or equal to 100 m/s, 90 m/s, 80 m/s, 70 m/s, 60 m/s, 50 m/s, 40m/s, 30 m/s, 20 m/s, 10 m/s, 5 m/s or 1 m/s.
 45. A mass spectrometer asclaimed in claim 41, wherein said first velocity is selected from thegroup consisting of: (i) 10-250 m/s; (ii) 250-500 m/s; (iii) 500-750m/s; (iv) 750-1000 m/s; (v) 1000-1250 m/s; (vi) 1250-1500 m/s; (vii)1500-1750 m/s; (viii) 1750-2000 m/s; (ix) 2000-2250 m/s; (x) 2250-2500m/s; (xi) 2500-2750 m/s; and (xii) 2750-3000 m/s.
 46. A massspectrometer as claimed in claim 43, wherein said second velocity isselected from the group consisting of: (i) 10-250 m/s; (ii) 250-500 m/s;(iii) 500-750 m/s; (iv) 750-1000 m/s; (v) 1000-1250 m/s; (vi) 1250-1500m/s; (vii) 1500-1750 m/s; (viii) 1750-2000 m/s; (ix) 2000-2250 m/s; (x)2250-2500 m/s; (xi) 2500-2750 m/s; and (xii) 2750-3000 m/s.
 47. A massspectrometer as claimed in claim 43, wherein said second velocity issubstantially the same as said first velocity.
 48. A mass spectrometeras claimed in claim 29, wherein said one or more transient DC voltagesor said one or more transient DC voltage waveforms has a frequency, andwherein said frequency: (i) remains substantially constant; (ii) varies;(iii) increases; (iv) increases then decreases; (v) decreases; or (vi)decreases then increases.
 49. A mass spectrometer as claimed in claim29, wherein said one or more transient DC voltages or said one or moretransient DC voltage waveforms has a wavelength, and wherein saidwavelength: (i) remains substantially constant; (ii) varies; (iii)increases; (iv) increases then decreases; (v) decreases; or (vi)decreases then increases.
 50. A mass spectrometer as claimed in claim 1,wherein two or more transient DC voltages or two or more transient DCvoltage waveforms pass simultaneously along said ion guide.
 51. A massspectrometer as claimed in claim 50, wherein said two or more transientDC voltages or said two or more transient DC voltage waveforms arearranged to move: (i) in the same direction; (ii) in oppositedirections; (iii) towards each other; or (iv) away from each other. 52.A mass spectrometer as claimed in claim 1, wherein one or more transientDC voltages or one or more transient DC voltage waveforms are repeatedlygenerated and passed in use along said ion guide, and wherein thefrequency of generating said one or more transient DC voltages or saidone or more transient DC voltage waveforms: (i) remains substantiallyconstant; (ii) varies; (iii) increases; (iv) increases then decreases;(v) decreases; or (vi) decreases then increases.
 53. A mass spectrometeras claimed in claim 1, further comprising an ion detector, said iondetector being arranged to be substantially phase locked in use withpulses of ions emerging from the exit of said ion guide.
 54. A massspectrometer as claimed in claim 1, further comprising a Time of Flightmass analyser comprising an electrode for injecting ions into a drift orflight region, said electrode being arranged to be energised in use in asubstantially synchronised manner with the pulses of ions emerging fromthe exit of said ion guide.
 55. A mass spectrometer as claimed in claim1, further comprising an ion trap arranged downstream of said ion guide,said ion trap being arranged to store and/or release ions from said iontrap in a substantially synchronised manner with pulses of ions emergingfrom the exit of said ion guide.
 56. A mass spectrometer as claimed inclaim 1, further comprising a mass filter arranged downstream of saidion guide, wherein a mass to charge ratio transmission window of saidmass filter is varied in a substantially synchronised manner with pulsesof ions emerging from the exit of said ion guide.
 57. A massspectrometer as claimed in claim 1, wherein said ion guide comprisesone, two, or more than two entrances for receiving ions and one, two, ormore than two exits from which ions emerge from said ion guide.
 58. Amass spectrometer as claimed in claim 1, wherein said inner electrode issubstantially Y-shaped.
 59. A mass spectrometer as claimed in claim 1,wherein said outer electrode is substantially Y-shaped.
 60. A massspectrometer as claimed in claim 1, wherein said ion guide comprises atleast one entrance for receiving ions along a first axis and at leastone exit from which ions emerge from said ion guide along a second axis,wherein said outer electrode and/or said inner electrode are curvedbetween said entrance and said exit.
 61. A mass spectrometer as claimedin claim 60, wherein said ion guide is substantially “S”-shaped and/orhas a single point of inflexion.
 62. A mass spectrometer as claimed inclaim 60, wherein said second axis is laterally displaced from saidfirst axis.
 63. A mass spectrometer as claimed in claim 1, wherein saidion guide comprises at least one entrance for receiving ions along afirst axis and at least one exit from which ions emerge from said ionguide along a second axis, wherein said second axis is inclined at anangle θ to said first axis and wherein θ>0°.
 64. A mass spectrometer asclaimed in claim 63, wherein θ falls within the range: (i)<10°; (ii)10-20°; (iii) 20-30°; (iv) 30-40°; (v) 40-50°; (vi) 50-60°; (vii)60-70°; (viii) 70-80°; (ix) 80-90°; (x) 90-100°; (xi) 100-110°; (xii)110-120°; (xiii) 120-130°; (xiv) 130-140°; (xv) 140-150°; (xvi)150-160°; (xvii) 160-170°; and (xviii) 170-180°.
 65. A mass spectrometeras claimed in claim 1, wherein at least a portion of said ion guideeither: (i) varies in size and/or shape along the length of said ionguide; or (ii) has a width and/or height which progressively tapers insize.
 66. A mass spectrometer as claimed in claim 1, wherein said innerelectrode is arranged offset from the central axis of said outerelectrode.
 67. A mass spectrometer as claimed in claim 1, wherein thedistance between said inner electrode and said outer electrode variesalong at least a portion of said ion guide.
 68. A mass spectrometer asclaimed in claim 1, further comprising an ion source, said ion sourcebeing selected from the group consisting of: (i) an Electrospray (“ESI”)ion source; (ii) an Atmospheric Pressure Chemical Ionisation (“APCI”)ion source; (iii) an Atmospheric Pressure Photo Ionisation (“APPI”) ionsource; (iv) a Matrix Assisted Laser Desorption Ionisation (“MALDI”) ionsource; (v) a Laser Desorption Ionisation (“LDI”) ion source; (vi) anInductively Coupled Plasma (“ICP”) ion source; (vii) an Electron Impact(“EI”) ion source; (viii) a Chemical Ionisation (“CI”) ion source; (ix)a Fast Atom Bombardment (“FAB”) ion source; and (x) a Liquid SecondaryIons Mass Spectrometry (“LSIMS”) ion source.
 69. A mass spectrometer asclaimed in claim 1, further comprising a pulsed ion source.
 70. A massspectrometer as claimed in claim 1, further comprising a continuous ionsource.
 71. A mass spectrometer as claimed in claim 1, said ion guidehaving an entrance for receiving ions and an exit from which ions arereleased, wherein said entrance and/or exit of the ion guide aremaintained at a potential so that ions are reflected at said entranceand/or exit.
 72. A mass spectrometer as claimed in claim 71, furthercomprising at least one ring lens, plate electrode or grid electrodearranged at said entrance and/or exit of said ion guide and wherein saidat least one ring lens, plate electrode or grid electrode is arranged tobe maintained at a potential so that ions are reflected at said entranceand/or exit.
 73. A mass spectrometer as claimed in claim 72, wherein anAC or RF voltage and/or a DC voltage is supplied to said at least onering lens, plate electrode or grid electrode so that ions are reflectedat said entrance and/or exit.
 74. A mass spectrometer as claimed inclaim 1, further comprising a mass analyser arranged downstream of saidion guide, said mass analyser selected from the group consisting of: (i)a Time of Flight mass analyser; (ii) a quadrupole mass analyser; (iii) aFourier Transform Ion Cyclotron Resonance (“FTICR”) mass analyser; (iv)a 2D (linear) quadrupole ion trap; (v) a 3D (Paul) quadrupole ion trap;and (vi) a magnetic sector mass analyser.
 75. A mass spectrometer asclaimed in claim 1, wherein in a mode of operation said ion guide ismaintained in use at a pressure selected from the group consisting of:(i) greater than or equal to 0.0001 mbar; (ii) greater than or equal to0.0005 mbar; (iii) greater than or equal to 0.001 mbar; (iv) greaterthan or equal to 0.005 mbar; (v) greater than or equal to 0.01 mbar;(vi) greater than or equal to 0.05 mbar; (vii) greater than or equal to0.1 mbar; (viii) greater than or equal to 0.5 mbar; (ix) greater than orequal to 1 mbar; (x) greater than or equal to 5 mbar; and (xi) greaterthan or equal to 10 mbar.
 76. A mass spectrometer as claimed in claim 1,wherein in a mode of operation said ion guide is maintained in use at apressure selected from the group consisting of: (i) less than or equalto 10 mbar; (ii) less than or equal to 5 mbar; (iii) less than or equalto 1 mbar; (iv) less than or equal to 0.5 mbar; (v) less than or equalto 0.1 mbar; (vi) less than or equal to 0.05 mbar; (vii) less than orequal to 0.01 mbar; (viii) less than or equal to 0.005 mbar; (ix) lessthan or equal to 0.001 mbar; (x) less than or equal to 0.0005 mbar; and(xi) less than or equal to 0.0001 mbar.
 77. A mass spectrometer asclaimed in claim 1, wherein in a mode of operation said ion guide ismaintained in use at a pressure selected from the group consisting of:(i) between 0.0001 and 10 mbar; (ii) between 0.0001 and 1 mbar; (iii)between 0.0001 and 0.1 mbar; (iv) between 0.0001 and 0.01 mbar; (v)between 0.0001 and 0.001 mbar; (vi) between 0.001 and 10 mbar; (vii)between 0.001 and 1 mbar; (viii) between 0.001 and 0.1 mbar; (ix)between 0.001 and 0.01 mbar; (x) between 0.01 and 10 mbar; (xi) between0.01 and 1 mbar; (xii) between 0.01 and 0.1 mbar; (xiii) between 0.1 and10 mbar; (xiv) between 0.1 and 1 mbar; and (xv) between 1 and 10 mbar.78. A mass spectrometer as claimed in claim 1, wherein a mode ofoperation said ion guide is maintained in use at a pressure selectedfrom the group consisting of: (i) greater than or equal to 1×10⁻⁷ mbar;(ii) greater than or equal to 5×10⁻⁷ mbar; (iii) greater than or equalto 1×10⁻⁶ mbar; (iv) greater than or equal to 5×10⁻⁶ mbar; (v) greaterthan or equal to 1×10⁻⁵ mbar; and (vi) greater than or equal to 5×10⁻⁵mbar.
 79. A mass spectrometer as claimed in claim 1, wherein in a modeof operation said ion guide is maintained in use at a pressure selectedfrom the group consisting of: (i) less than or equal to 1×10⁻⁴ mbar;(ii) less than or equal to 5×10⁻⁵ mbar; (iii) less than or equal to1×10⁻⁵ mbar; (iv) less than or equal to 5×10⁻⁶ mbar; (v) less than orequal to 1×10⁻⁶ mbar; (vi) less than or equal to 5×10⁻⁷ mbar; and (vii)less than or equal to 1×10⁻⁷ mbar.
 80. A mass spectrometer as claimed inclaim 1, wherein in a mode of operation said ion guide is maintained, inuse, at a pressure selected from the group consisting of: (i) between1×10⁻⁷ and 1×10⁻⁴ mbar; (ii) between 1×10⁻⁷ and 5×10⁻⁵ mbar; (iii)between 1×10⁻⁷ and 1×10⁻⁵ mbar; (iv) between 1×10⁻⁷ and 5×10⁻⁶ mbar; (v)between 1×10⁻⁷ and 1×10⁻⁶ mbar; (vi) between 1×10⁻⁷ and 5×10⁻⁷ mbar;(vii) between 5×10⁻⁷ and 1×10⁻⁴ mbar; (viii) between 5×10⁻⁷ and 5×10⁻⁵mbar; (ix) between 5×10⁻⁷ and 1×10⁻⁵ mbar; (x) between 5×10⁻⁷ and 5×10⁻⁶mbar; (xi) between 5×10⁻⁷ and 1×10⁻⁶ mbar; (xii) between 1×10⁻⁶ mbar and1×10⁻⁴ mbar; (xiii) between 1×10⁻⁶ and 5×10⁻⁵ mbar; (xiv) between 1×10⁻⁶and 1×10⁻⁵ mbar; (xv) between 1×10⁻⁶ and 5×10⁻⁶ mbar; (xvi) between5×10⁻⁶ mbar and 1×10⁻⁴ mbar; (xvii) between 5×10⁻⁶ and 5×10⁻⁵ mbar;(xviii) between 5×10⁻⁶ and 1×10⁻⁵ mbar; (xix) between 1×10⁻⁵ mbar and1×10⁻⁴ mbar; (xx) between 1×10⁻⁵ and 5×10⁻⁵ mbar; and (xxi) between5×10⁻⁵ and 1×10⁻⁴ mbar.
 81. A mass spectrometer comprising an ion guide,said ion guide comprising a guide wire, cylindrical or rod electrode andan outer cylindrical electrode wherein, in use, both an AC and a DCpotential difference is maintained between said guide wire, cylindricalor rod electrode and said outer cylindrical electrode.
 82. A method ofmass spectrometry, comprising: guiding ions along an ion guidecomprising an outer electrode and an inner electrode disposed withinsaid outer electrode; maintaining said inner and outer electrodes at aDC potential difference such that ions experience a first radial forcetowards said inner electrode; and applying an AC or RF voltage to saidinner and/or said outer electrodes so that ions experience a secondradial force towards said outer electrode.
 83. A mass spectrometercomprising: an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst saidions are being transported axially through said ion guide.
 84. A massspectrometer as claimed in claim 83, wherein said guide wire comprises asemiconductor or resistive wire so that an axial DC field is maintained,in use, along said ion guide by the application of a DC voltage betweenthe ends of said guide wire.
 85. A mass spectrometer comprising: an ionguide comprising a guide wire held centrally in a plurality of outerconcentric cylindrical electrodes wherein both AC and DC voltages areapplied, in use, between the guide wire and the plurality of outerconcentric cylindrical electrodes in order to radially retain ionswhilst said ions are being transported axially through said ion guide.86. A mass spectrometer as claimed in claim 85, wherein an axial DCfield is maintained, in use, along said ion guide by the application ofDC voltages to said plurality of outer cylindrical electrodes.
 87. Amass spectrometer as claimed in claim 85, wherein travelling potentialwave functions are applied, in use, to said outer cylindrical electrodesto assist in ion transmission.
 88. A mass spectrometer comprising: anion guide comprising a guide wire held centrally in an electricallyconductive cylindrical tube electrode wherein both AC and DC voltagesare applied, in use, between the guide wire and the cylindrical tubeelectrode and wherein ions are arranged, in use, to impact the insidewall of said cylindrical tube electrode or the guide wire to producesecondary ion disassociation by adjusting the AC or DC voltages.
 89. Amass spectrometer comprising: an ion guide comprising a guide wire heldcentrally in an electrically conductive cylindrical tube electrodewherein both AC and DC voltages are applied, in use, between the guidewire and the cylindrical tube electrode and wherein said AC voltage orsaid DC voltage is adjusted so as to cause an increase in the internalenergy of ions within said ion guide thereby inducing collisionalfragmentation or collisional induced disassociation of said ions.
 90. Amass spectrometer comprising: an ion guide comprising an innercylindrical electrode held centrally in an electrically conductivecylindrical tube electrode wherein both AC and DC voltages are applied,in use, between the inner cylindrical electrode and the cylindrical tubeelectrode in order to radially retain ions whilst said ions are beingtransported axially through said ion guide.
 91. A mass spectrometercomprising: an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst saidions are being transported axially through said ion guide and whereinsaid guide wire splits into two or more wires.
 92. A mass spectrometeras claimed in claim 91, wherein different AC or DC voltages are appliedto said two or more wires.
 93. A mass spectrometer comprising: an ionguide comprising a guide wire held centrally in an electricallyconductive cylindrical tube electrode wherein both AC and DC voltagesare applied, in use, between the guide wire and the cylindrical tubeelectrode in order to radially retain ions whilst said ions are beingtransported axially through said ion guide and wherein said guide wireis not straight.
 94. A mass spectrometer as claimed in claim 93, whereinsaid guide wire is circular.
 95. A mass spectrometer comprising an ionguide, said ion guide comprising a Y-shaped outer cylindrical electrodeand a Y-shaped inner guide wire electrode, wherein in use said outerelectrode and said inner electrode are supplied with both an AC voltageand a DC voltage and wherein said ion guide is arranged so that an ionbeam is split or ion beams are joined.
 96. A mass spectrometercomprising: an ion guide comprising a guide wire held centrally in anelectrically conductive cylindrical tube electrode wherein both AC andDC voltages are applied, in use, between the guide wire and thecylindrical tube electrode in order to radially retain ions whilst saidions are being transported axially through said ion guide, said ionguide further comprising a ring lens, plate or grid and wherein anadditional DC or AC voltage is applied, in use, to said ring lens, plateor grid so that ions are reflected backwards and are trapped or storedwithin said ion guide.